4
Counting faces and edges
Learner’s Book 1: Unit 2, pp. 40–41
GoalSay how many faces, edges and curved surfaces a cube, a sphere and a cylinder have, and which flat shape fits on a face.
New words
face · a flat side of a 3D shapeedge · where two faces meetsurface · the outside of a shape, flat or curved
Explanation
Let us count. A cube has 6 flat faces and 12 edges. A sphere has no flat faces and no edges. It has 1 curved surface. A cylinder has 2 flat faces and 1 curved surface. It has 2 edges, where the flat faces meet the curved surface. A flat face of a cube is a square, so a cube fits on a square. A flat face of a cylinder is a circle. A sphere has no flat face, so it does not fit onto a flat shape.
| cube | sphere | cylinder | |
|---|---|---|---|
| flat faces | 6 | 0 | 2 |
| curved surfaces | 0 | 1 | 1 |
| edges | 12 | 0 | 2 |
Worked examples
A cube has 6 flat faces: one on the top, one on the bottom and 4 around the sides.
A sphere has 0 edges and 0 flat faces. It has only one curved surface.
A cylinder has 2 flat faces and 1 curved surface. A cylinder fits onto a circle.
Class activity
Tower challenge: with boxes, tins and a ball, build the tallest tower you can. As you add each piece, say how many flat faces it has and whether it has edges. Where does the ball have to go so the tower does not fall? Then use your finger to count the faces of a box.
Practice
1
How many flat faces does a cube have?
6
2
How many edges does a sphere have?
0
3
How many flat faces does a cylinder have?
2
4
Which flat shape fits onto one face of a cube: the square or the circle?
the square